The present invention relates to a liquid crystal display, and more particularly to a liquid crystal display with compensated pixel arrays.
A liquid crystal display is a thin and flat display device comprised of a number of pixels arrayed in front of a light source or reflector. Each pixel contains a layer of liquid crystal molecules between two electrodes. The liquid crystal molecules have electric charges on them. Applying biases to the electrodes creates electrostatic forces that twist the molecules. This twists the light passing through the molecules, and allows varying degrees of light to pass (or not to pass) through the filters. An image can therefore be displayed by those rows and columns of pixels.
FIG. 1 illustrates a conventional pixel array 100, in which each pixel 102 is formed by a first sub-pixel 104 and a second sub-pixel 106 divided by a slit 120 therebetween. In order to improve viewing angle characteristics, the first and second sub-pixels 104 and 106 are designed with different threshold voltages, such that the two sub-pixels 104 and 106 would be charged at different saturation voltage levels when they illuminate. The different saturation voltage levels of the two sub pixels 104 and 106 cause the liquid crystal molecules therein to have different orientation directions. Thus, this improves the viewing angle characteristics.
In order to further improve the viewing angle characteristics, each sub-pixel 104 is divided into a number of domains, in which the liquid crystal molecules have various orientation directions. For example, the sub-pixel 104 is divided by protrusions 108 and 110 into three portions. Due to the geometry of the three portions, the molecules of the upper-left portion have an orientation direction represented by an arrow 112, the molecules of the lower-left portion have an orientation direction represented by an arrow 114, and the molecules of the right portion have one orientation direction represented by an arrow 116 for its upper half and another orientation direction represented by an arrow 118 for its lower half. Each portion of an orientation direction defines a domain. Thus, the sub-pixel 102 has four domains.
These various domains improve the viewing angle characteristics. FIG. 2 illustrates a cross-sectional view 200 of the sub-pixel 104 along line A-A. When the electrodes 202 and 204 are charged, the protrusion 108 causes the molecules at the right to orient along one direction, and the molecules at the left to orient along another direction. This allows the top position 206, upper-right position 208, and upper left position 210 to receive the same amount of light. In other words, the sub-pixel 104 can be viewed from various angles with relatively uniform light transmittance.
One drawback of the conventional pixel array 100 is that the protrusions and slits reduce its aperture ratio, which refers to the ratio between the area of a pixel that can transmit light and the actual area of the pixel. It is understood by people skilled in the art that more protrusions and slits lead to a lower aperture ratio. As shown in FIG. 1, the sub-pixels 104 and 106 are divided by a slit 120, and each of them includes two protrusions. This reduces the aperture ratio of the pixel 102.
FIG. 3 illustrates another conventional pixel array 300, in which each pixel 302 is formed by a first sub-pixel 304 of a lower threshold voltage and a second sub-pixel 306 of a higher threshold voltage. As shown in the drawing, each sub-pixel 304 or 306 only has one protrusion. Thus, the aperture ratio of the pixel 302 is improved.
One drawback of the conventional pixel array 300 is that its pixel arrangement is often susceptible to the “mura” issue, which refers to the non-uniformity of an image over a large area of pixels. Referring to FIGS. 3 and 4 simultaneously, when viewing from the right side of the conventional pixel array 300, certain rows of sub-pixels 402 would appear to be darker because of their molecule orientations. The sub-pixel 306 has a higher threshold voltage, so that it appears to be darker when it illuminates. The sub-pixel 304 has a lower threshold voltage, so that it appears to be brighter when it illuminates. As a result, columns 404 would appear to be darker because they are formed by the high threshold voltage sub-pixels 306, while columns 406 would appear to be brighter because they are formed by low threshold voltage sub-pixels 304. This causes bright and dark stripes interwoven with each other, which is the “mura” defect.
As such, what is needed is a liquid crystal display with a pixel array that provides a high aperture ratio, while being free from the “mura” defect.